The invention relates to a self-starting two-pole single-phase synchronous motor with a permanent-magnetic rotor, which rotor as a result of diametrical magnetization has two opposite magnetic poles faces on its circumference and is arranged between two limbs of a stator provided with an exciter coil, the ends of these limbs partly enclosing the rotor with arcuate stator pole faces arc, so that air gaps are formed.
Such a motor, whose principle is for example described in DE-AS 14 88 270, should be capable of providing a maximum power output with a high efficiency, while guaranteeing that the rotor starts correctly from its rest position. To ensure reliable starting it is necessary that the relevant rest position of the rotor differs from the angular positions of the rotor in which the driving torque passes through the value zero. These rotor positions, in which the driving torque is zero, occur when its magnetic field is directed transversely of the limbs of the stator, so that two such rotor positions, are obtained, which positions are 180.degree. spaced from each other in accordance with the diametrical magnetization but which are perfectly equivalent, because they only differ in respect of the orientation of the magnetic field. Consequently, such a rotor also has two rest positions, which are 180.degree. spaced and which are also perfectly equivalent.
The angular deviation of the rest positions of the rotor, from those positions in which the driving torque passes through the value zero, is obtained in a known manner by designing the ends of the stator limbs so that different air gaps relative to the rotor are formed. For example, asymmetrically angularly displaced projections may be formed on the ends of the limbs. In conjunction with the magnetic field of the rotor this yields a so-called detent torque, which ensures that when the motor is disconnected from the a.c. line voltage the rotor occupies one of the two desired rest positions and does not remain in one of the two positions in which the driving torque passes through the value zero, which would prohibit self-starting.
In order to meet the desired power, efficiency and starting requirements, the rotors of such self-starting two-pole single-phase synchronous motors, which are for example employed in domestic appliances, such as citrus presses and the like, are manufactured from sintered anisotropic magnetic materials with a high residual induction, because the value of this induction is a measure of both the power output and the detent torque which can be achieved. Such sintered anisotropic magnetic materials, however, have the disadvantage that they are deformed substantially during sintering, so that rotors thus manufactured should be subjected to a finishing process. This process requires comparatively expensive grinding operations, because such materials are very hard. As a result of this, such synchronous motors become relatively expensive, and the cost restricts their use, especially in mass products.